Support for unsigned `Fixed`-point operations

[appetrosyan]

Sometimes it is obvious that a number cannot be negative (or zero). It is also true of fixed point numbers.

Given this, it would be nice if Fixed supported using the extra bit for the precision, e.g.

• Fixed<u64> which itself starts from zero, and doesn't carry the sign bit. Internally promotes to u128 just like i64.
• Fixed<NonZeroU64>. Same as before except it also has a niche.

The implementations for cadd, csub etc. need to be adjusted accordingly.

[loyd]

I've thought about it, and it would be a nice feature, but I'm afraid to complicate things while all FixedPoint instances are generated by a macro. I want to reconsider this idea after moving to const generics.